Heretofore, an art of processing the outer shape of a can shell for storing beverages, foods etc. to improve the design performance of the can shell by recess-deforming the can shell and forming a three-dimensional pattern thereto is known.
Upon performing this type of outer shape processing, for example, a pair of receive molds is inserted to the interior of a cylindrical can shell from openings formed on both sides of the can shell, by which a molding portion corresponding to the shape of the recess deformation is formed by the confronting width between the ends facing each other of the pair of receive molds. On the other hand, a pressure roller is applied to press the area corresponding to the mold portion from the outer side of the can shell. Then, the can shell is rotated while maintaining the pressing operation by the pressure roller, by which the whole circumference of the can shell is recess-deformed.
However, when the recess-deformation is formed using the pressure roller and the receive mold, the wall thickness of the recessed portion is reduced due to the draw deformation by the pressure roller and the receive mold, by which the strength of the can shell is disadvantageously deteriorated.
Further, when performing this type of outer shape processing by inserting a receive mold into the can shell, the receive mold contacts and slides against the inner surface of the can shell and may generate scratches on the inner surface of the can shell, and especially if the inner surface of the can shell is coated with a coating or the like, may damage the coating. Furthermore, by using a receive mold, the shape of the receive mold may remain on the can shell, which may deteriorate the appearance of the three-dimensional pattern.
Moreover, if a can lid is crimped onto one end of the can shell, or if a bottom portion is integrally formed to the cylindrical portion as in a so-called two-piece can shell, the receive mold can only be inserted from the opening portion at one end of the can shell, which may cause a drawback in that a desired recess shape cannot be obtained.
Furthermore, since it is necessary to have the positions of the pressure roller and the receive mold correspond accurately, the device configuration became complex, which disadvantageously increased the price of the device and increased the manufacture costs.
Therefore, another prior art method is known in which a can shell is placed inside an outer die having a three-dimensional pattern formed on its inner side, a molding head equipped with a rubber expansion unit that is expandable toward the outer circumferential direction is inserted to the interior of the can shell, and the expansion unit is expanded by water pressure to press the can shell against the inner surface of the outer die and to process the three-dimensional pattern on the inner surface of the outer die to the outer surface of the can shell. According to this method, since the rubber expansion unit comes into contact with the inner surface of the can shell, the inner surface of the can shell can be prevented from being damaged.
According to this method, however, since the expansion unit is expanded to expand the can shell and to form a pattern on the can shell, there is a drawback in that outer shape processing aimed at shrinking the diameter of the can shell cannot be performed. Further, the molding head must have a complex structure since it must have in addition to the expansion unit a flow path for supplying water to the expansion unit and so on, and even further, the can shell must be expanded by applying extremely high pressure to the expansion unit so as to press the can shell against the inner surface of the outer die, so the cost of the device becomes expensive, and the manufacture cost is disadvantageously increased. Furthermore, since the can shell is deformed by the pressure by the rubber expansion unit applied from the inner side of the can shell, even if it is desirable to form plural relatively close recess portions on the outer surface of the can shell, for example, there is a drawback in that the recessed portions cannot be formed sufficiently on the outer surface of the can shell.
In order to solve the drawbacks mentioned above, the present invention aims at providing a method and device for processing the outer shape of a can shell that prevents the strength of the can shell from deteriorating and also reliably prevents the inner surface of the can shell from being scratched or the coating from being damaged, that enables outer shape processing to be performed to even can shells having one end closed, and that enables outer shape processing with improved design performance to be easily performed at low costs and without complicating the device configuration.